Reciprocating piston machines of the type to which the present invention relates typically include a central shaft journalled in a block and a plurality of cylinders disposed parallel to the shaft and located on the block at angularly-spaced intervals circumferentially about the shaft. The shaft includes an oblique or Z-crank portion on which is journalled a nutating spider or "wobble plate." The spider has radially extending branches linked at swivel joints by means of rods to reciprocating pistons located in the cylinders.
The machines may be used as compressors, in which case the shaft is driven; or may be used as power generators or engines, in which case a fluid under pressure is injected or formed within the cylinders to drive the pistons. When the machine is constructed as an internal combustion engine, the fluid takes the form of gasoline, diesel fuel or other combustible fluid which is exploded at the end of a compression stroke to cause the pistons to be driven. The thrust of the driven pistons is imparted to the spider by means of the rods while spider rotation is restrained, thereby causing nonrotational nutation of the spider which is translated into rotation of the shaft. A common arrangement has two pistons in each cylinder, with oppositely directed rods that join separate spiders which cooperate in synchronization to drive the shaft.
Examples of known nutating spider crank reciprocating piston machines are given in U.S. Pat. Nos. 1,808,380; 3,007,462; 3,212,483; 3,528,317; and 4,489,682. All such arrangements have separate, opposing single-acting pistons with oppositely directed rods that reach into distal ends of the pistons to link the pistons to terminal ends of the radial branches of separate spiders. Two (typically ball and socket) swivel connections are used, one at the junctures of the pistons with the rods, and one at the junctures of the rods with the spiders. Means, such as a spline or guide rail arrangements, are required to absorb the rotational torque and restrain the spider from rotating with the shaft. Such machines are often referred to as "barrel" engines because of their revolver barrel-like appearance.